Thin-film hard disc magnetic media are widely used for on-line data storage in computers. In recent years, considerable effort has been spent in achieving higher recording density. Among the most important factors in this recording density are:
(1) Magnetic remanence, which is a measure of the signal amplitude which can be read from an isolated pulse stored in the medium-the greater the remanence, or moment, the greater the signal amplitude which can be detected in a reading operation. PA1 (2) Coercivity, defined as the magnetic field required to reduce the remanence magnetic flux to 0, i.e., the field required to erase a stored bit of information. Higher coercivity in a medium allows adjacent recorded bits to be placed more closely together without mutual cancellation. Thus, higher coercivity is associated with higher information storage density. PA1 (3) Signal/noise ratio, defined as the ratio of the power of a given signal to the noise power in a given bandwidth, and provides a measure of signal amplitude relative to noise level. The higher the S/N ratio, the greater the bit density which can be read with a given degree of reliability. PA1 (4) Overwrite, defined as the effectiveness in erasing a signal read at one frequency with a higher frequency signal. Overwrite provides a measure of the residual signal which remains after overwrite by a new signal. PA1 (5) Flying height, i.e., the distance which a read/write head floats above the spinning disc. Less overlaps of voltage signals in adjacent magnetic domains in the disc occurs as the read/write head is moved closer to the disc surface, allowing recording density to be increased. The flying height is limited principally by surface irregularities in the disc. For a thin-film medium formed on standard aluminum substrate, the flying height is about 6 microinches. Flying heights as low as 1-2 micro-inches can be achieved in a thin-film medium formed on more perfect-surface substrate, such as a glass substrate.
In addition, it is now a common feature in hard disc systems to vary the read-write sampling frequency according to the radial position of the read-write head on the disc. In particular, the sampling frequency is increased, in progressing toward the outer-diameter region of the disc, to maintain a substantially constant linear bit density. Thus, for example, if the linear disc speed at the outer disc diameter is twice that at the inner diameter, the sampling frequency should also be twice as great at the outer diameter region to give the same linear density of bits at both inner- and outer-diameter regions. This greater sampling frequency results in a higher signal output and higher noise at the outer diameter of the disc.
At the same time, the overwrite characteristics in a thin-film medium tend to suffer at the outer diameter region of a thin-film medium. This is because of the higher recording frequency and because the head flying height, which depends on planing effects, increases with the greater disc linear velocity on moving toward the outer diameter region of the disc. Thus, a flying height of 6 micro-inches at the inner disc diameter can result in a 12 micro-inch flying height at the outer disc diameter. The greater flying height at the outer-diameter region, along with the higher recording frequency, reduces the ability of the head flux to saturate the magnetic domains in the magnetic layer, thus reducing overwrite characteristics.
In summary, radial-position effects due to differences in sampling rate and flying height at different radial positions on a disc can significantly effect the performance characteristics of the disc, particularity S/N ratio and overwrite, imposing different bit density limitations at both the inner- and outer-diameter regions of the disc.
It would therefore be desirable to provide a magnetic thin-film medium having a low signal-to-noise ratio at the inner diameter region of the medium, and good overwrite characteristics at the outer diameter region of the disc.